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Howards Preso / leiden 2004 (fwd)

From: Dirk-Willem van Gulik <dirkx@asemantics.com>
Date: Fri, 23 Apr 2004 05:19:21 -0700 (PDT)
To: public-rdf-dawg@w3.org
Message-ID: <20040423051901.R54360@skutsje.san.webweaving.org>

XQuery: a whirlwind tour

Howard Katz


data model


atomic values and nodes (book prize for number of types!) are injected
into (== constructed in) an empty data model

XQuery is a functional language: expressions (Expr "+" Expr) (eg additive
expression) evaluate their operands and return results, with expressions
higher up the query tree do the same thing, taking the data model instance
through a series of transformations for each expression we "ascend"

compositionality says same thing: we want the output of any expression to
be able to act as an input to the higher-up expression calling it.
Syntactically, for binary operators (eg) reflected in the grammar. ie,
operands in following can be just about anything :

	AdditiveExpr ::= MuliplicativeExpr ( ("+" | "-")
MultiplicativeExpr )*

So in general, for case of generic


	   /     \

	expr	expr

it's never a syntax error for operands to be *any* type of expression (tho
it may produce other types of errors -- see below)

final query result is whatever emerges through the top of the query tree,
the last man standing

so data model instances consist of an ordered, heterogenous sequence of
"items": atomic values and/or nodes (so one of differences between XQuery
Data Model and InfoSet is ordered forests, as well as PSVI from XML Schema

*everything* is a sequence, even one-offs (singleton sequences)



	1 	(does this look like xml to you??)

		construct an IntegerLiteral 1 in the query

		1 goes in -- 1 comes out


	"2", 3 + 1


	   /	            \

       	"2"    		    "+"

			/         \

		IntegerLiteral	IntegerLiteral

	rule for evaluating :

	construct a sequence containing a single xsd:string "2"

	construct two atomic xs:integer values 3 and 1

	add the latter 2 together -> 4

	concatenate the 4 onto the "2"

		=> 42


	<foo/>, <bar/>

		=> <foo/><bar/>

		(again, not quite xml)


       query tree for more complex expression

       1 + count( doc( "bib.xml" )//text() )


          Additive [ + ]

             IntegerLit [ 1 ]


                QName [ count ]

                RelPath [ // ]


                      QName [ doc ]

                      StringLit [ bib.xml ]


easy way to implement all the above is to have every expression evaluated
via a function call that returns a generic sequence of items to the
function that calls it. ie signature is something like

	ItemSequence additiveExpr( 	ItemSequence additiveExprLHS,

					ItemSequence additiveExprRHS )

my implementation returns a sequence of two-element integer arrays

	int[][] additiveExpr( int[][] additiveExprLHS, int[][]
additiveExprRHS )

principle of full compositionality can lead to interesting syntactic
constructs. eg looking at addition:

	AdditiveExpr ::= MuliplicativeExpr ( ("+" | "-")
MultiplicativeExpr )*

	MultiplicativeExpr := UnaryExpr ( "*" | "div" | "idiv" | "mod" )
UnaryExpr )*


we can say

	1 + <foo>abc</foo> 	(: RIPLEY QUERY #1 :)

similarly, any step in xpath/location path can be a generic Expr:

	<foo><bar/></foo>/bar  	(: RIPLEY QUERY #2 :)

Grammar is complex


 o 142 productions (nov03 working draft)

 o 320 terminals(+/-)

 o heavy use of lexical states:






we start in DEFAULT state and change state according to state-change rules
in main spec document

eg one such rule:

   if in START_TAG state:

   "/>"               popState()

   "<"                ELEMENT_CONTENT

   " ' "              QUOT_ATTRIBUTE_CONTENT

   " " "              APOS_ATTRIBUTE_CONTENT

   "{}" DEFAULT       pushState()

   S, QName, "="      no-op

some sequence stuff


(10, (1, 2), (), (3, 4)) 	=> (10, 1, 2, 3, 4) => 101234

(10, 1 to 4)			=> (10, 1, 2, 3, 4) => 101234

(21 to 29)[ 5 ]

fn:reverse( 1 to 3) 	=> (3, 2, 1)

3 possible outcomes for evaluation of any XQuery expression


1. produce a valid result

2. produce a null sequence result

3. throw an error

case-by-case. eg, here's what we do w/ arithmetic operators:

   1. apply atomization to each operand (rule for producing a single
atomic value from a sequence)

   2. If either resulting sequence is (), => ()

   3. if fn:count( seq_LHS ) or fn:count( seq_RHS ) > 1, throw a type

   4. if either sequence is now of type xdt:untypedAtomic, cast to the
default type for the operator.

      (default for all operators except idiv is xs:double. If the cast
fails, throw a dynamic error

   5. apply the operator. either return the result or throw a dynamic


	1 + <foo>abc</foo>

fails under rule #4. throw a dynamic error

	1 + <foo>123</foo>


	=> 124



general comparisons, inherited from XPath 1.0:

	42 = 42	       => true (: not unexpected :)

what happens with a general comparison where one operand or the other is a
sequence ??

	doc( "bib.xml" )//book[ author/last = "Kennedy" ]

ie, what if the book has 3 authors, equivalent to :

	( "Abiteboul", "Buneman", "Suciu" ) = "Suciu"

answer: we do "existential comparison


	(1, 2) = (2, 3 )  (: RIPLEY QUERY #3 :)

General comparison is not transitive

	(1, 2) = (2, 3) => true

	(2, 3) = (3, 4) => true

	(3, 4) = (4, 5) => true


	(1, 2) = (4, 5) => false

Question: how to inject existing nodes into the Data Model? ("traditional
function of an XML query language):

input functions


so called because they input selected nodes into the data model

we usually use to root location paths -- connect them to source documents
of interest

	doc( "bib.xml" )//book

	collection( "someUri" )/bib/book/author[ 1 ]

these location path return node in document order

what happens *across* documents is impl-dependent but stable during a
session (@@@@@ CHECK THIS @@@@@)

constructed nodes


we can also inject/construct our own nodes

	<foo>I <i>am</i> handsome!</foo>

node construction gives XQuery rich ability to do transformations and
"rich" reporting, replacing much of XSLT

Much ado about query analysis vs. query evaluation leads to distinction
between static errors and dynamic errors, static vs dynamic type checking

constructed nodes are first-class citizens, not just decoration. *all*
nodes have identity

	<foo/> is <foo/> => false	(: RIPLEY QUERY #4 :)

wrapping "real" nodes in an element constructor is more than cosmetic. we
need to deep copy:



		doc( "bib.xml" )//editor


because of need for disambiguation of editor/.. operation

	<foo>{ (: evaluate what goes here :) } </foo>


	<foo>{ doc( "bib.xml" )//node() }</foo>

What about (Straing

	<foo>{ <bar> { <baz/> } </bar> } </foo>

user-defined functions


    define function is-document-element( $e as element() ) as xs:boolean


        if ( $e/.. instance of document-node() )

        then true()

        else false()


depending on how invoked :

    is-document-element( doc( "bib.xml" )/bib )     => true

    is-document-element( doc( "bib.xml" )//editor ) => false

    is-document-element( doc( "bib.xml" )//book )   => throws

To note :

variation #3 throws because signature calls for *single* element

alternate signatures might be

  $e as element()?

  $e as element()*

  $e as element()+

  $e as element( foaf:Person )

  $e as element( price, xsd:decimal )

"rich" prolog structure standardizes switches and context-setters inside
the query


   define variable $titles { doc(
®)//title };

   define function outtie($v as xs:integer) as xs:integer external;

   define variable $v as xs:integer external;

   declare namespace foo = "http://example.org";

   <foo:bar> { doc( "http://fooThings.com" )//foo:bar } </foo:bar>

flwors return tuples


    for $number in ( 1, 3, 5 )

    	for letter in ( "A", "B", "C" )

	return ( $number, "-", $letter, " " )

    => 1-A  1-B  1-C  3-A  3-B  3-C  5-A  5-B  5-C

sample queries


some $emp in //employee satisfies ($emp/bonus > 0.25 * $emp/salary)

  o existential semantics == "any"

  o as opposed to every $emp in //employee ...




    for $b in doc("http://www.bn.com/bib.xml")//book

    where $b/publisher = "Addison-Wesley" and $b/@year > 1991

    order by $b/title



            { $b/@year }

            { $b/title }






    <book year="1992">

        <title>Advanced Programming in the Unix environment</title>


    <book year="1994">

        <title>TCP/IP Illustrated</title>



   o order by changes ordering on demand from default document order


for $s in doc("report1.xml")//section[section.title = "Procedure"]


	($s//instrument)[ position()<=2 ]


for $p in doc("report1.xml")//section[section.title = "Procedure"]

where not( some $a in $p//anesthesia satisfies

        $a << ($p//incision)[1] )

return $p




    for $s in doc("book.xml")//section

    let $f := $s/figure


        <section title="{ $s/title/text() }" figcount="{ count($f) }"/>



xpaths vs flwors


XPaths navigate, select nodes en bulk, tho predicates (filters) can narrow
the size of the result list (not result set btw)

flwrs allow finer-grained manipulation of individual items, additional
navigation from selected nodes (for), complex multi-level (re)grouping

	o fine-grained manipulation of individual items

	o grouping and decoration of semi-results (ie "reporting")

	o complex transformation

short-circuitable error reporting


    declare function func( $arg_1 as item? ) as item*


	(: something :)


    some $x in expr() satisfies $x = funct( $x )

is allowed to return true immediately if found, even if possibility of
erroring on subsequent items

documentation, all at W3C XML Query website (sorry, no url)


[ Howard's Top 4 ]

 o XQuery 1.0: An XML Query Language (http://www.w3.org/TR/xquery/) [132
pages, wd 12nov03: last call]

	The main overview document. Describes main features and surfaced
language syntax

 o XML Query Use Cases (http://www.w3.org/TR/xquery-use-cases) [84 pages,
wd 12nov03]

	Specific enought to be used for test cases. Good sample of actual
XQuery snippets

 o XQuery 1.0 and XPath 2.0 Data Model
(http://www.w3.org/TR/xpath-datamodel) [67 pages, wd 12nov03; last call]

	XML Schema Datatypes Part II + 7 node types, their properties, and

	DM = XML Infoset + ordered forests + XML Schema PSVI

 o XQuery 1.0 and XPath 2.0 Functions and Operators
(http://www.w3.org/TR/xpath-functions/) [166 pages, wd 12nov03; last call]

	+/- 150 built-functions


XML Query (XQuery) Requirements
(http://www.w3.org/TR/xquery-requirements/) [14 pages, wd 12nov03]

XQuery 1.0 and XPath 2.0 Formal Semantics
(http://www.w3.org/TR/xquery-semantics) [175 pages, wd 20feb03; last call]

XML Syntax for XQuery 1.0 (XQueryX) (http://www.w3.org/TR/xqueryx/) [52
pages, wd 19nov03]

XSLT 2.0 and XQuery 1.0 Serialization
(http://www.w3.org/TR/xslt-xquery-serialization/) [20 pages, wd 12nov03;
last call]

XPath Requirements Version 2.0 (http://www.w3.org/TR/xpath20req/) [17
pages, wd 22aug03]

XQuery and XPath Full-Text Requirements
(http://www.w3.org/TR/xquery-full-text-requirements) 10 pages, wd 2may03]

XQuery and XPath Full-Text Use Cases
(http://www.w3.org/TR/xmlquery-full-text-use-cases/) [106 pages, wd

834 pages in total

other suggestions


 o hands-on via saxon at sourceforge (sorry, no url)

 o "XQuery from the Experts", ed Howard Katz, 2003

 o "Practical XQuery", Mike Brundage, AW, 2004



 o functional language is elegant, easy to implement

 o support for xml makes it complex

 o support for typed xml makes it more complex

 o user-defined functions (either internal or external) provide

 o great to be able to move numerous "declare" switches inside the query

 o ability to do ad hoc grouping via node constructors w/ arbitary
sequencing provides rich transformational/reporting capability (another
way to say is the ability order user-defined heterogeneous sequences and
optionally annotate them is powerful)
Received on Friday, 23 April 2004 08:26:41 UTC

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